Protein Kinases
Kinases regulate many different cell proliferation, differentiation, and signaling processes by adding phosphate groups to proteins. Uncontrolled signaling has been implicated in a variety of disease conditions including inflammation, cancer, arteriosclerosis, and psoriasis. Reversible protein phosphorylation is the main strategy for controlling activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate, which drives activation, is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by protein kinases and removed from that protein by protein phosphatases. Phosphorylation occurs in response to extracellular signals (hormones, neurotransmitters, growth and differentiation factors, etc), cell cycle checkpoints, and environmental or nutritional stresses and is roughly analogous to turning on a molecular switch. When the switch goes on, the appropriate protein kinase activates a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, or transcription factor.
The kinases comprise the largest known protein group, a superfamily of enzymes with widely varied functions and specificities. They are usually named after their substrate, their regulatory molecules, or some aspect of a mutant phenotype. With regard to substrates, the protein kinases may be roughly divided into two groups; those that phosphorylate tyrosine residues (protein tyrosine kinases, PTK) and those that phosphorylate serine or threonine residues (serine/threonine kinases, STK). A few protein kinases have dual specificity and phosphorylate threonine and tyrosine residues. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The N-terminal domain, which contains subdomains I-IV, generally folds into a two-lobed structure, which binds and orients the ATP (or GTP) donor molecule. The larger C terminal lobe, which contains subdomains VI A-XI, binds the protein substrate and carries out the transfer of the gamma phosphate from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue. Subdomain V spans the two lobes.
The kinases may be categorized into families by the different amino acid sequences (generally between 5 and 100 residues) located on either side of, or inserted into loops of, the kinase domain. These added amino acid sequences allow the regulation of each kinase as it recognizes and interacts with its target protein. The primary structure of the kinase domains is conserved and can be further subdivided into 11 subdomains. Each of the 11 subdomains contains specific residues and motifs or patterns of amino acids that are characteristic of that subdomain and are highly conserved (Hardie, G. and Hanks, S. (1995) The Protein Kinase Facts Books, Vol I7-20 Academic Press, San Diego, Calif.).
The second messenger dependent protein kinases primarily mediate the effects of second messengers such as cyclic AMP (cAMP), cyclic GMP, inositol triphosphate, phosphatidylinositol, 3,4,5-triphosphate, cyclic-ADPribose, arachidonic acid, diacylglycerol and calcium-calmodulin. The cyclic-AMP dependent protein kinases (PKA) are important members of the STK family. Cyclic-AMP is an intracellular mediator of hormone action in all prokaryotic and animal cells that have been studied. Such hormone-induced cellular responses include thyroid hormone secretion, cortisol secretion, progesterone secretion, glycogen breakdown, bone resorption, and regulation of heart rate and force of heart muscle contraction. PKA is found in all animal cells and is thought to account for the effects of cyclic-AMP in most of these cells. Altered PKA expression is implicated in a variety of disorders and diseases including cancer, thyroid disorders, diabetes, atherosclerosis, and cardiovascular disease (Isselbacher, K. J. et al. (1994) Harrison's Principles of Internal Medicine, McGraw-Hill, New York, N.Y., pp. 416-431, 1887).
Calcium-calmodulin (CaM) dependent protein kinases are also members of STK family. Calmodulin is a calcium receptor that mediates many calcium regulated processes by binding to target proteins in response to the binding of calcium. The principle target protein in these processes is CaM dependent protein kinases. CaM-kinases are involved in regulation of smooth muscle contraction (MLC kinase), glycogen breakdown (phosphorylase kinase), and neurotransmission (CaM kinase I and CaM kinase II). CaM kinase I phosphorylates a variety of substrates including the neurotransmitter related proteins synapsin I and II, the gene transcription regulator, CREB, and the cystic fibrosis conductance regulator protein, CFTR (Haribabu, B. et al. (1995) EMBO Journal 14:3679-86). CaM II kinase also phosphorylates synapsin at different sites, and controls the synthesis of catecholamines in the brain through phosphorylation and activation of tyrosine hydroxylase. Many of the CaM kinases are activated by phosphorylation in addition to binding to CaM. The kinase may autophosphorylate itself, or be phosphorylated by another kinase as part of a “kinase cascade”.
Another ligand-activated protein kinase is 5′-AMP-activated protein kinase (AMPK) (Gao, G. et al. (1996) J. Biol Chem. 15:8675-81). Mammalian AMPK is a regulator of fatty acid and sterol synthesis through phosphorylation of the enzymes acetyl-CoA carboxylase and hydroxymethylglutaryl-CoA reductase and mediates responses of these pathways to cellular stresses such as heat shock and depletion of glucose and ATP. AMPK is a heterotrimeric complex comprised of a catalytic alpha subunit and two non-catalytic beta and gamma subunits that are believed to regulate the activity of the alpha subunit. Subunits of AMPK have a much wider distribution in non-lipogenic tissues such as brain, heart, spleen, and lung than expected. This distribution suggests that its role may extend beyond regulation of lipid metabolism alone.
The mitogen-activated protein kinases (MAP) are also members of the STK family. MAP kinases also regulate intracellular signaling pathways. They mediate signal transduction from the cell surface to the nucleus via phosphorylation cascades. Several subgroups have been identified, and each manifests different substrate specificities and responds to distinct extracellular stimuli (Egan, S. E. and Weinberg, R. A. (1993) Nature 365:781-783). MAP kinase signaling pathways are present in mammalian cells as well as in yeast. The extracellular stimuli that activate mammalian pathways include epidermal growth factor (EGF), ultraviolet light, hyperosmolar medium, heat shock, endotoxic lipopolysaccharide (LPS), and pro-inflammatory cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1).
PRK (proliferation-related kinase) is a serum/cytokine inducible STK that is involved in regulation of the cell cycle and cell proliferation in human megakaroytic cells (Li, B. et al. (1996) J. Biol. Chem. 271:19402-8). PRK is related to the polo (derived from humans polo gene) family of STKs implicated in cell division. PRK is downregulated in lung tumor tissue and may be a proto-oncogene whose deregulated expression in normal tissue leads to oncogenic transformation. Altered MAP kinase expression is implicated in a variety of disease conditions including cancer, inflammation, immune disorders, and disorders affecting growth and development.
The cyclin-dependent protein kinases (CDKs) are another group of STKs that control the progression of cells through the cell cycle. Cyclins are small regulatory proteins that act by binding to and activating CDKs that then trigger various phases of the cell cycle by phosphorylating and activating selected proteins involved in the mitotic process. CDKs are unique in that they require multiple inputs to become activated. In addition to the binding of cyclin, CDK activation requires the phosphorylation of a specific threonine residue and the dephosphorylation of a specific tyrosine residue.
Protein tyrosine kinases, PTKs, specifically phosphorylate tyrosine residues on their target proteins and may be divided into transmembrane, receptor PTKs and nontransmembrane, non-receptor PTKs. Transmembrane protein-tyrosine kinases are receptors for most growth factors. Binding of growth factor to the receptor activates the transfer of a phosphate group from ATP to selected tyrosine side chains of the receptor and other specific proteins. Growth factors (GF) associated with receptor PTKs include; epidermal GF, platelet-derived GF, fibroblast GF, hepatocyte GF, insulin and insulin-like GFs, nerve GF, vascular endothelial GF, and macrophage colony stimulating factor.
Non-receptor PTKs lack transmembrane regions and, instead, form complexes with the intracellular regions of cell surface receptors. Such receptors that function through non-receptor PTKs include those for cytokines, hormones (growth hormone and prolactin) and antigen-specific receptors on T and B lymphocytes.
Many of these PTKs were first identified as the products of mutant oncogenes in cancer cells where their activation was no longer subject to normal cellular controls. In fact, about one third of the known oncogenes encode PTKs, and it is well known that cellular transformation (oncogenesis) is often accompanied by increased tyrosine phosphorylation activity (Carbonneau H and Tonks NK (1992) Annu. Rev. Cell. Biol. 8:463-93). Regulation of PTK activity may therefore be an important strategy in controlling some types of cancer.
LIM Domain Kinases (LIMKs)
The novel human protein provided by the present invention is a novel LIM domain kinase (LIMK) alternative splice form. The novel LIMK splice form of the present invention shares the highest degree of sequence similarity with LIMK1, isoform 1 (see the amino acid sequence alignment of SEQ ID NO:2 with LIMK1 in FIG. 2).
LIMK1 contains a protein kinase domain in the C-terminal half of the protein, and two repeats of cysteine-rich LIM/double zinc finger motifs at the N-terminus, with an intervening proline-serine-rich region that contains casein kinase and map kinase recognition sites (Mizuno et al., Oncogene 1994 June; 9(6):1605-12). The LIM domain is thought to participate in protein-protein interactions by binding to other LIM motifs. LIM domains are highly conserved cysteine-rich structures that contain 2 zinc fingers. Approximately 40 eukaryotic LIM-containing proteins are known to exist. Because the LIM domain is often present in the homeodomain-containing transcriptional regulators and oncogenic nuclear proteins, LIMK1 is thought to play a role in developmental or oncogenic processes through interactions with such LIM-containing proteins (Mizuno et al., Oncogene 1994 June; 9(6):1605-12). LIMK1 is also thought to be a component of an intracellular signaling pathway and is thought to be involved in brain development (Tassabehji et al., Nature Genet. 13: 272-273, 1996). Consistent with this involvement in the brain, virtual northern blot analysis indicates expression of the present novel LIMK splice form in brain neuroblastoma cells and hypothalamus tissue (see FIG. 1)
In addition to its association with oncogenic processes, intracellular signaling, brain development, LIMK has specifically been found to be deleted in Williams syndrome, and LIMK1 is likely to be responsible for the unexplained neurologic features of Williams syndrome (Tassabehji et al., Nat Genet 1996 July; 13(3):272-3). Furthermore, LIMK1 hemizygosity has been implicated in the impaired visuospatial constructive cognition of Williams syndrome (Frangiskakis et al., Cell 86: 59-69, 1996).
Additionally, LIMK1 has been identified as a potent activator of serum response factor (SRF; involved in regulating transcription of numerous serum-inducible and muscle-specific genes). SRF activation by LIMK1 depends on the ability of LIMK1 to regulate actin treadmilling (Sotiropoulos et al., Cell 98: 159-169, 1999).
LIMK is phosphorylated and activated by ROCK, which is a downstream effector of Rho. LIMK phosphorylates cofilin, thereby reducing the actin-depolymerizing activity of cofilin. This pathway leads to Rho-induced reorganization of the actin cytoskeleton (Maekawa et al., Science 285: 895-898, 1999).
Thus, it is clear that novel LIMK proteins and alternative splice forms, particularly those related to LIMK1, have valuable commercial utilities related to the treatment and diagnosis of such pathologies as brain/neurological disorders, cancers, and disorders associated with serum response factor.
For a further review of LIMKs, see Bernard et al., Genomics 35: 593-596, 1996; Maekawa et al., Science 285: 895-898, 1999; Mao et al., Cytogenet. Cell Genet. 74: 190-191, 1996; Okano et al., J. Biol. Chem. 270: 31321-31330, 1995; Proschel et al., Oncogene 11: 1271-1281, 1995; Osborne et al., Genomics 36 (2), 328-336 (1996); and Edwards et al., J. Biol. Chem. 274 (16), 11352-11361 (1999).
Kinase proteins, particularly members of the LIM domain kinase subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of kinase proteins. The present invention advances the state of the art by providing previously unidentified human kinase proteins that have homology to members of the LIM domain kinase subfamily.